1. Field of the Invention
The present invention generally relates to a secondary air supplier for use in a vehicular engine. More particularly, the present invention pertains to a diagnostic system for diagnosing the condition of the secondary air supplier. The air supplier supplies secondary air to an exhaust passage of the engine, in order to purify the exhaust gas.
2. Description of the Related Art
Conventional exhaust gas purification techniques generally utilize an oxygen sensor and a three way catalytic converter, in order to satisfy two requirements. The first requirement is to satisfy the regulatory rules, and the other requirement is to minimize the engine fuel consumption. These conventional techniques include oxidizing and/or deoxidizing the carbon monoxide (CO), hydro-carbon (HC) and nitrogen oxides (NOx), contained in the exhaust gas, in order to purify it. In order to efficiently complete these reactions, the air-fuel ratio in the engine should be maintained at about a theoretical air-fuel ratio. The concept for maintaining the theoretical air-fuel ratio has been embodied in several engine systems.
An exemplary engine system of this type includes an engine control unit, an oxygen sensor disposed along an exhaust passage, and a secondary air supplier for directly supplying the secondary air from an intake passage to the exhaust passage. The control unit calculates the air-fuel ratio (A/F) of the air-fuel mixture in the engine cylinders based on the output signal from the oxygen sensor. Further, the control unit executes a closed loop control operation (i.e., feedback control operation) for controlling the supplied amount of air and/or fuel. As a result of this control, the air-fuel ratio approaches the theoretical air-fuel ratio.
In order to execute the above-described closed loop control operation, a feedback correction factor (FAF) is utilized for calculating a target supplied amount of air and/or fuel. The feedback correction factor (FAF) could vary in relation to the detected result of an oxygen sensor. While the air-fuel ratio (A/F) is controlled to approach the theoretical air-fuel ratio, the feedback correction factor (FAF) alters, where the value "1.0" is the center of variation.
When the engine is operating under specific conditions, the secondary air is supplied to the exhaust passage by means of the secondary air supplier, in order to warm up the catalyst (i.e., in order to activate the catalyst).
The following are illustrative specific operation conditions of the engine: a condition wherein the coolant temperature is still cold; and a condition wherein. The control unit executes an open loop control operation for the air-fuel ratio (A/F), while the secondary air is being supplied. The control unit re-executes the closed loop control operation for the air-fuel ratio simultaneously with the termination of the secondary air supply.
In this conventional engine system, the exhaust emission could become faulty when the secondary air supplier malfunctions.
The Japanese Unexamined Patent Publication No. 63-143362 proposes a diagnostic system for diagnosing whether or not the secondary air supplier is functioning properly. In this system, the secondary air supplier is forcibly activated when the closed loop control operation for controlling the amount of fuel injected is executed. The secondary air is supplied to the exhaust passage by means of the secondary air supplier. Further, a mean value of the feedback correction factor (FAF) will be calculated. When the calculated mean value becomes smaller than a fundamental value ("1.0") for a predetermined period of time, the diagnostic system determines that the secondary air supplier is malfunctioning.
However, in the engine system which is executing the closed loop control operation for the amount of fuel injected, when the engine is operating in a constant speed condition, the feedback correction factor (FAF) approaches the fundamental value "1.0". When a vehicle is varying its engine condition (e.g. decelerating), the feedback correction factor (FAF) might largely shift away from the fundamental value "1.0".
In other words, the amount of fuel injected by the fuel injector is calculated in the following way: a fundamental amount of fuel injected is determined by taking a ratio (intake amount/engine speed), and is adjusted by the feedback correction factor (FAF), which is determined in relation to the result detected by the oxygen sensor. The amount of the air intake is a value measured by an air flowmeter. Accuracy of measurement by this air flowmeter tends to vary specifically, when the amount of air intake is decreased in response to a change of operating conditions of the engine, from the constant speed operating condition to the decelerating condition. Therefore, while the vehicle is decelerating, the feedback correction factor (FAF) might largely shift away from the fundamental value "1.0".
Therefore, if the secondary air is supplied while the feedback correction factor temporarily and largely shifts away from the fundamental value "1.0", the mean value of the feedback correction factor, for use in a diagnostic operation, might become smaller than the predetermined value for a predetermined period of time. According to the conventional technology which simply compares the mean value of the feedback correction factor (FAF) with the predetermined value, the diagnostic system might erroneously determine that the secondary air supplier is malfunctioning, even when the secondary air supplier is properly functioning.